"Slug-pulling" is a recurring problem in the operation of high speed punch presses. "Slugs" are the pieces of metal that are punched from workpieces in a punch press operation; "slug-pulling" refers to the tendency of slugs to follow the punch head in its return stroke, the slug ending up at or near the plane of the workpiece and giving rise to a variety of problems as the workpiece is moved quickly past the punch and die in subsequent punching operations. The slug-pulling problem is particularly severe when small-sized punches and dies are employed, and can cause considerable down time and loss of production. The latter is particularly evident upon consideration of the fact that modern day punch presses often operate in the range of from about 200 to about 400 strokes per minute.
Historically, the die opening dimensions are set at values exceeding the dimensions of the hole to be punched in a workpiece by about 8% of the workpiece thickness. One finds that the diameter of the hole thus punched increases from the punch side to the die side of the workpiece, the punch side of the hole having dimensions of the punch and the die side of the hole having the dimensions of the die opening. The slugs that are formed accordingly have overall dimensions that are larger than the punch side dimensions of the punched hole, and it is not uncommon, as a slug follows the punch during the return stroke of the punch, for the slug to become wedged or lodged in the hole from which it was punched, thereby interferring with subsequent movement of the workpiece. It is often necessary to resharpen the punch tips, and it has been found that the propensity of punch tips to become dulled decreases if the die opening dimensions are increased so as to exceed the dimensions of the hole to be punched in the workpiece by as much as 20% of the workpiece thickness. The resulting increase in die opening dimensions also serves to accentuate the dimensional differences between the punch side and die side of a punched hole, thereby rendering the slug-pulling problem more severe.
Various means have been adopted to retard or prevent the slug-pulling problem. Punches themselves may be provided with centrally positioned slug-ejector rods for the purpose of separating the slug from the face of the punch head and ejecting the slug from the die during each punching cycle. Because of the expense involved in manufacturing punch heads with separately moveable ejector pins, and because of the rather poor results obtained with such devices, this solution to the slug-pulling problem has not gained wide popularity.
Another solution involves the use of a heavy grease or the like within the die to adhere the slug to the die and prevent it from following the punch head during the punch head's return stroke. This solution to the slug-pulling problem has not gained wide popularity.